The use of intraluminal prosthetic devices has been demonstrated to present an alternative to conventional vascular surgery. Intraluminal prosthetic devices are commonly used in the repair of aneurysms, as liners for vessels, or to provide mechanical support and prevent the collapse of stenosed or occluded vessels.
Intraluminal endovascular prosthetics involve the percutaneous insertion of a generally tubular prosthetic device, such as a stent, into a vessel or other tubular structure within the vascular system. The stent is typically delivered to a specific location inside the vascular system in a low profile (pre-deployed) state by a catheter. Once delivered to the desired location, the stent is deployed by expanding the stent into the vessel wall. The expanded stent typically has a diameter that is several times larger than the diameter of the stent in its compressed state. The expansion of the stent may be performed by several methods known in the art, such as by a mechanical expansion device (balloon catheter expansion stent) or by self-expansion.
Preferably, a stent would possess a minimum width and wall thickness, which should minimize thrombosis at the stent site after implantation. The preferred stent would also possess sufficient hoop strength to resist elastic recoil of the vessel. Many current tubular stents employ a multiplicity of circumferential sets of strut members connected by either straight longitudinal connectors or undulating longitudinal connecting connectors in an effort to fulfill the above requirements.
The strut members, of which there are ordinarily a plurality that extend around the circumference of the device, can be formed from a number of diagonal sections in turn connected to curved or arced members reminiscent of elbows, thereby forming a zig-zagging structure in a closed ring arrangement. When expanded, the stent provides structural support for the vessel wall. Strut members may be formed from a single piece of metal having a uniform wall thickness and generally uniform strut width. The curved members are formed having a generally uniform wall thickness and generally uniform width.
While the geometry of the stent members is uniform, under load, the strain experienced by each strut member is not. The “stress” applied to the stent across any particular cross section is the force per unit area. These dimensions are those of pressure, and are equivalent to energy per unit area. The stress applied to the stent includes forces experienced by the stent during deployment, and comprises the reactive force per unit area applied against the stent by the vessel wall. The resulting “strain” (deformation) that the stent experiences is defined as the fractional extension perpendicular to the cross section under consideration.
During deployment and in operation, each strut member experiences varying load along its length. High stress and/or strain can cause cracking of the metal and potential fatigue failure of the stent under the stress of a beating heart. It should also be remembered that arteries “pulse” at typically 70 times per minute or more, about 40 million times per year—necessitating that these devices are designed to last in excess of 108 loading cycles for a 10-year life. Thus, guarding against cyclic fatigue failure is a particularly important consideration in stent design. Designs can be physically tested and analytically evaluated to ensure acceptable stress and strain levels are achievable based on physiologic loading considerations. This is typically achieved using the traditional stress/strain-life (S-N) approach, where design and life prediction rely on a combination of numerical stress predictions as well as experimentally-determined relationships between the applied stress or strain and the total life of the component. Fatigue loading for the purpose of this description includes, but is not limited to, axial loading, bending, torsional/twisting loading of the stent, individually and/or in combination. One of skill in the art would understand that other fatigue loading conditions can also be considered.
A bifurcation is a location where the vessel divides into two branches or parts, that is, a main branch vessel and a side branch vessel. One, two, or both branches may exhibit a curvature or bend. The vessel bifurcations generally have circumferential asymmetry. That is, bifurcated vessels generally exhibit asymmetry around their circumference at the point where the main vessel divides into one or more branches. Thus, the opening in the side branch vessel where the side branch vessel joins the main branch vessel may be asymmetrical. The side branch vessel may join the main branch vessel at an oblique angle, which may contribute to the asymmetry of the side branch opening.
In any event, a bifurcation or bend in a vessel can present challenges if an implant is to be deployed there. Where the implant needs to be in a specific orientation (such as for maximizing the therapeutic effect, such as to conform to the bend in one of the main branch or side branch vessels, it would be helpful if the implant were flexible over at least a portion of its surface, so that the device could conform to the bend.
A medical implant, such as a stent, which has circumferential regions that exhibit a relatively high degree of flexibility when compared to other circumferential regions of the device, to exhibit relatively increased flexibility in at least one bending direction while also providing a relative increased degree of stiffness in another bending direction, would be advantageous and advance the state of the art. Such an arrangement, provided for in the implant, would allow the stent to preferentially bend in at least one direction, so the device may conform to curves in the vessels as it traverses in its crimped state on the way to the deployment site, or otherwise in its deployed state conform to the geometry of the vessel at the deployment site, if the implant is deployed at a bend. Likewise, flexibility and conformability are advantageous where the vessel has lesions that render the interior vessel configuration nonlinear.